Loudspeaker enclosure



April 4, 1961 A. H. ROBERTS 2,978,060

LOUDSPEAKER ENcLosURE Filed VApril s, 1959 INVENTO /I fred/M Rogerex-(12 w a Freya/amy (6,06) 2 ATTORNEY United States Patent O 2,978,060LOUDSPEAKER ENCLOSURE Alfred H. Roberts, 1615 Monk Road, Gladwyne, Pa.Filed Apr. 3, 1959, Ser. No. 803,856 3 Claims. (Cl. 181-31) Thisinvention relates to acoustical devices and more particularly to loudspeaker mountings and enclosures.

In seeking faithful reproduction of voice and music, acousticaldesigners have devoted much effort to the lowest three octaves'of theaudible spectrum, encompassing 16 to 128 cycles per second.

The free-air resonant frequency of most loudspeakers of the familiarelectromagnetic (dynamic) type lies within this bass portion of theaudible spectrum.

Above this resonant frequency, as is known, the diaphragm of such aspeaker is mass-controlled and means of coupling it to the air arerelatively easy to devise. Below this resonant frequency, the diaphragmis stiliness-controlled and means of coupling it to the air arerelatively diflicult to devise.

With a large room space available, the horn is acknowledged as the bestacoustic coupler in the present state of the art. A horn is a structurehaving a throat, a

mouth, and an enclosed intervening air column so tapered as to present aresistive impedance to the driver diaphragm over a range of frequenciesdown to a lower limit, called the cutolf frequency. The horn is anacoustical transformer, matching a driver of high impedance at thethroat (sound pressure divided by volume velocity) to a low impedance atthe mouth.

At any cross section of a perfect horn, an acoustic impedance matchexists; the impedance is resistive throughsoA out the length of thehorn; and there are no reflec- Y tions (standing waves) from mouth orthroat.

All finite horns are, however, imperfect. With each imperfection in theflare rate and each deficiency in size of the mouth, the impedancebecomes reactive; standing waves arise in the air column; and thefrequency response is irregular.

The organ pipe, or untapered air column, is another method of coupling atransducer to its acoustical environment. Longitudinal reflections(standing waves) occur in a straight pipe in a regular harmonic series.A pipe closed at one end and open at the other will speak or resonate atfrequencies corresponding to odd multiples of standing quarter-waveswithin the pipe. That is, the pipe will have a fundamental pitch havinga Wave length four times that of the pipe, and harmonics arise at 3f,5f, 7 f, etc., where f is the fundamental frequency.

On the other hand, a pipe closed at both ends or open at both ends willspeak at a fundamental pitch corresponding to a Wave length twice thatof the pipe, and at the even harmonics thereof, viz., 2f, 4f, 6j, etc.

It is important to note that, 'to a transducer located in the closed endof a pipe which is open at `the other end, a standing wave isantiresonant; that is, 'at f, 3f, 5f, etc., the air column behaves likean electrical parallel resonant circuit and the impedance presented tothe driver reaches a maximum.

Thus a wave of rarefaction traveling down the tube from the diaphragm isreflected back from the mouth as avwave of pressure (i.e., reversed inphase at the mouth) next pressure wave front down the tube. In otherwords, the vibrating diaphragm and the reflected wave move in antiphaseand tend to buck each other out at f, 3f, 5f, etc.

The action of the quarter-wave tube or pipe as described above isclosely analogous to a quarter-Wave radio transmission line shortcircuited at one end, often called a quarter-wave stub. Each acts as animpedance transformer, inverting the pressure-current ratios at theends.

The'constants of a pipe, i.e., acoustic mass, compliance and resistance,are said to be distributed along its length. For example, in thequarter-wave stub pipe just cited, the impedance near the closed end,viewed toward the closed end, is a compliance; the compliant air iscornpressed as by a bicycle pump. Looking out the open end, theimpedance is a mass; the air moves back and forth as a plug; it isaccelerated but not compressed.

Equations for the tube given in acoustical textbooks (Acoustics, L. L.Beranek, McGraw-Hill Book Co., 1954, pp. 131 et seq.) are confined totubes with not too great a diameter, in terms of Wavelength; otherwisetransverse resonances must be taken into account. The radius in meters,it is stated, should be less than about At c.p.s. this radius would be61/2 inches, the diameter 13 inches.

A third device for coupling a transducer to the air is the ventedenclosure, known variously as bass-reflex enclosure, Helmholtzenclosure, acoustic filter, acoustic phase inverter, or distributed-portenclosure (see A. L. Thuras, Sound Translating Device, U.S. Patent No.1,869,178, Frank Robbins and William Joseph, Acoustic System forLoudspeakers, U.S. Patent No. 2,694,463, Jordan J. Baruch and H. C.Lang, Some Vented Enclosures for Loudspeakers, Acoustics Lab., M.I.T.,January 1953).

The Helmholtz resonator combines a nearly pure compliance, in the formof `a volume of air confined within a sphere or box which is small interms of wave length, with a nearly pure mass, a plug of air within aduct or tube, very short compared to a wave length. The duct is attachedto the enclosure. At resonance the inductive reactance of the masscancels the capacitive reactance of the compliance, in a manner wellknown in acoustical and electrical alternating-current theory.

In the bass-reex enclosure, a dynamic loudspeaker is mounted on one faceof the box, usually adjacent to the port or duct, and the area of thelatter is made equal to or smaller than the effective area of thespeaker diaphragrn. There are innumerable variations.

and it hits the diaphragm just as the -latter is pushing the 1.

It is interesting to note that the Thuras Patent No. 1,869,178 showsthirteen tubular ducts three inches in r diameter and six inches inlength, ranged around a loudspeaker having a -resonant frequency ofabout 55 c.p.s., Iall contained in a box of approximately thirteen cubicfeet. Among the enclosures of this basic type at present beingmanufactured in the United States, there are many having a single ductapproximating three inches in diameter and six inches in length, butperhaps of different shape, a loudspeaker of approximately the sameresonant frequency, and a volume of one to two cubic feet. With thereduction in size there has been a corresponding reduction intranslation efficiency.

A brief comparison of the three aforementioned types of air couplers ispertinent. The horn is an acoustical transformer having an impedancematch throughout its length. The transformation ratio, i.e., impedanceratio, is established by the relative cross-sectional areas of mouth andthroat; the radiation resistance is determined by the area .of themouth.V The straight cylindrical or aaraoeo rectangular pipe exhibits avarying complex impedance along its length, the impedance becomingresistive at resonance or antiresonance. The quarter-wave pipe closed atone end can act as an impedance transformer; but only nearantiresonance, when the impedance is resistive, can it radiatesubstantial acoustic power from the open end. The bass-reflex enclosure,unlike the pipe, has lumped mass and lumped compliance. It has only oneantiresonance (i.e., no harmonics) and like the quaterwave pipe,radiates usefully fromy tlieduct principally at and near antiresonance.These are broad generalities, but they are believed useful in connectionwith an understanding of the present invention.

In accordance with the present invention an, air coupler is providedwhich has certain properties (a), of a horn, (b) of a Helmholtzresonator or'bass-reiiex enclosure, and (c) of a pipe folded once,butwhich does not satisfy all the criteria for any one of the threetypes of' couplers referred to above.

It is the principal object of thepresen-t iiufentionr to provide aloudspeaker enclosure having an increasedl response at the lower audiblefrequencies, an improved response at the middle audible frequencies, anda diffusion of the output at the higher audible frequencies.

It is a further object of they present invention to p-rovide anacoustical device which is economical, in its embodiments, of labor,materialsand living space.

1It is a further object of the present invention to provide anacoustical device which is faithful in its reproduction of speech andmusical sounds.

It is a further object of the present invention to provide an acousticaldevice which is efficient in the, reproduction of sound.

It is a further object ofV the present invention to provide anacoustical device'which will function well with loudspeakers of diverseoperational properties.

`It is a further object of the present invention to provide anacoustical device which is adapted to unitary or modular constructionand which is adaptable to medium and high power sound projection, whenarrayed or stacked in multiple, without the use of auxiliary horns,reflectors or baffles, save the wall, ceiling or floor nearby.

Other objects and advantageous features of the invention will beapparent from the description and claims. The nature and characteristicfeaturesl of the invention will be more readily understood from thefollowing description, taken in connection with the accompanyingdrawings forming part thereof, in which:

Figure l is a front elevational view of an acoustical device inaccordance with the present invention and positioned against a verticalwall and opening towards the ceiling;

Fig. 2 is a vertical central sectional view taken approximately on theline 2-2 of Fig. 1;

Fig. 3 is a view of an acoustical device in accordance with the presentinvention positioned against a vertical wall and in spaced relation to ailoor;

Fig. 4 is a perspective view of an acoustical device in accordance withthe present invention positioned in the corner of a room and atintersecting walls;

Fig. 5 is an enlarged side elevational view of a speaker having adamping ring carried thereby;

Fig. 6 is a front view of the structure shown in Fig. S;

Fig. 7 is a diagrammatic View showing the impedancefrequency curve ofthe structure in accordance with the present invention; and

Fig. 8 is a diagrammatic view of the acoustical circuit with approximateelectrical analogs.

It should, of course, be understood that the description and drawingsherein are illustrative merely, and that various modifications andchanges can be made in the structure disclosed without departing fromthe spirit `of the invention.

Like numerals refer to like parts throughout thesev-l eral views.

Referring now more particularly to Figs. 1 and 2 of the drawings, inwhich a preferred embodiment is illustrated, the enclosure in accordancewith the invention in a practical and useful embodiment can have anexterior width of the order of l0 to 16 inches, an exterior depth of theorder of 16 inches, and a height in feet of the order of where fo of thespeaker for a conventional S inch speaker is 60 c.p.s.

The enclosure includes a horizontal bottom wall panel 10, spacedparallel vertical side wall panels 11 and 12, -a vertical rear wallpanel 13 which is adapted to be disposed against a vertical wall 14 of aroom or other enclosure, and a vertical front wall panel 15. A top wallpanel section 16 is provided horizontally disposed and connecting theside wall panels 11 and 12 and the front wall panel 15. A baie ormounting board 17 is provided extending downwardly frorn the panelsection 16 and between the side wall panels 11 and 12. The mountingboard 17 is rearwardly inclined, preferably at an angle of the order ofl5 degrees.

The wall panels 10 to 13, inclusive, can be made of any desired materialsuch as wood, plywood, chipboard, pressed cellulose board, laminatedplastic and the like, and with an approximate panel thickness of threefourths of an inch. The construction is rigid and the joints betweenmeeting wall panels are effectively air-tight. VAn interior covering offibrous material 1S, such as glass bers, mineral Wool, textile fibers orthe like, is provided, ofa thickness of the order of one inch on theinterior of the front wall panel 15, side wall panels 11 and 12, rearwall panel 13 and bottom wall panel l1t).

The baille or' mounting board 17 preferably has an opening 19therethrough for the diaphragm of a speaker 20 which is secured to themounting board 17 in any desired manner. The speaker 20 has a dampingring 21 mounted thereon asl hereinafter more fully explained.

The speaker 2i) can be of vany desired type such as electro magnetic ordynamic, as desired.. Mid-range or treble speakers can also be employed,an auxiliary speaker 22 being shown as mounted to face the listener. Thespeaker 20 has one of the surfaces 26a of its diaphragm or cone facingin one direction at the opening 19, and the other surfacey 20!) thereofoppositely disposed.

The space or cavity Within the wall panels has a throat portion. 30, anenergy sink 31 communicating therewith, and a mouth portion 32 extendingfroml the sink 31 with the terminus 31211 of the mouth portion 312.substantially in the plane of the wall panel section 16.

In Fig. 3, the enclosure structure is shown as turned so that the wallpanel 10a provides a top wall, and short legs 23 are provided, which canbe of a height of the order of five inches, to support the structurewith the mouth facing the floor. Y

`In the form of the invention illustrated in Fig. 4 in place'of a fiatrear wall panel 143, a pair of rear wall panels 13a and 13b areprovidedv so that the structure can be placed in the corner of a roomwith` the mouth facedupwardly as in Figs. l and 2.

From the foregoing it will be seen that the enclosure structure may beemployed in any desired arrangement with the rear wall panel 13 paralleland closely positioned with respect to aroom enclosing wall and with themouth 32 up or down, or if desired in any other direction.

The enclosure structure as shown may be employed as a single unit, inpairs or in multiple for stereophonic sound reproduction, and separatedat the desired distances to provide the desired effect in the particularroom enclosure in which the same are employed.

As illustrated in Fig. 2, a hard surfaced rellector plate 33 can beprovided along the top of the back or rear wall panel 13 for a dilfusionof higher frequencies in a manner known in the art.

Reference will now be made to the acoustical properties of the enclosurein accordance with the invention.

This enclosure in accordance with the invention de-v pends upon thequarter-wave action of a folded air column to establish an antiresonanceat and near the freeair resonant frequency of a dynamic loudspeaker. Inthis respect this invention resembles a pipe, folded once.

To achieve a maximum area and consequent radiation resistance at themouth, the baille board 17 for the speaker 20 is tilted backward atapproximately 15 degrees, as previously stated and as shown in Figure 2.The throat 30 behind the speaker 20 is four to six times smaller incross-sectional area than the mouth 32 of the coupler. This relationshipestablishes a transformationratio between mouth 32 and throat 30 forbetter resistive loading of the back of the speaker diaphragm. In thisyrespect the enclosure in accordance with the present invention resemblesa horn.

To realize the maximum mutual radiation impedance between the mouth 32of the coupler and the front of the speaker diaphragm, the speaker 20 isplaced within the mouth 32 in one embodiment shown in Figs. 1 and 2.Below the baille plate 17 and bounded by wall panels 10, 11, 1'2, 13 and15 there exists a compliant volume of air in the cavity or sink 31 whichfunctions as an energy sink at and near antiresonance. In these respectsthe enclosure structure in accordance with the invention resembles aHelmholtz resonator, or bass-reflex enclosure.

The enclosure in accordance with the invention does I not satisfy allthe criteria for any one of the aforementioned three types of coupler.It is not a horn because of the large reactive impedance existing in thecavity or sink 31. It is also not a horn because of the shortness of theair column with respect to the wave lengths transmitted. It is not apipe because of the large reactive impedance existing in the cavity orsink 31. IIt is not a Helmholtz resonator because the antiresonancefrequency of the device is not controlled by the cross-sectional area atany point, but rather by the length of the air column.

The compliance referred to and other acoustical parameters of theacoustical device in accordance with the i11- vention are shown in Fig.8 in which the speaker parameters are designated as follows:

C3-compliance of speaker suspension Rs--resistance of suspension plusradiation resistance of `front of diaphragm M3-mass of speaker voicecoil, diaphragm and air in front of diaphragm Rd-dissipative resistanceintroduced at back of diaphragm by damping ring 21 (to be describedlater herein)l lActing through the network, under pressure P3, is volumevelocity U3.

The coupler proper is represented electrically and acoustically by theshunt compliances C1 at the throat 30, C2 at the energy sink 31, and C3in the mouth 32, together with the masses M1, M2 and M3. Each branchtakes a branch current of volume velocity U1, U2 and U3. The useful loadon the air column, i.e., radiation impedance, yis represented by Rawithin a border. Rl and Rd vary with the second power of the frequency.

'1" he entire network, as shown in Fig. 8 is in effect a low-pass wavefilter composed of T sections, employed to match the air load at themouth 32 of the coupler` to that at the throat 30 at the back of thespeaker 20.

At antiresonance, when the resistive impedance at the mouth 32 isfalling rapidly, the driver diaphragm encounters an impedance peak whichlimits its excursion, reduces harmonic distortion arising from anon-linear speaker suspension and maximizes radiation at the mouth 32.This is similar to the action of the bass-reflex except that the mouth32 here resembles a horn, rather than a port or duct.

-Since Rd is falling with frequency, along with R., a

rough impedance match is maintained over a broader band of frequenciesthan hitherto possible in a coupler having lumped constants.

Above antiresonance, the shunt compliances C2, C2 and C3 progressivelyshortcircuit the back radiation of the speaker 20 so that the air columnbecomes acoustically opaque. This effect is enhanced by the increase ofR4 with frequency. Thus at middle and high sound frequencies the frontof the speaker 20 must do all the work, and the best balance is achievedbetween the output of the first three octaves (16 to 128 c.p.s.) and theoutput above 128 c.p.s., with the dimensions previously mentioned(approximately 12 inches by 16 inches by 24 inches) the upper cutofffrequency of the low-pass filter is computed to be about c.p.s.

Due -to the end effect of the air column, especially when the mouth 32radiates along a plane surface, the antiresonant frequency is lowered.`In a rectangle of side elevation 24 by 16 inches, the total folded aircolumn is approximately 2 X 24 plus 6 inches=54 inches against a wall oroor. The antiresonance then occurs at approximately 63 c.p.s. This iswell-related to many ldynamic loudspeakers of medium stiffness.

Just below the antiresonancetfrequency of the quarterwave air column andthe speaker (series-resonant branch of the network), the impedance atthe mouth 32 of the coupler becomes inductive. Likewise, the complianceswithin the coupler (C1, C2, C3) fade away. This leaves, in effect, aninductive air column, with vestigial radiation resistance atits outerend, to resonate with the compliance of the speaker surround. The lastuseful radiation at the low end, therefore, is accomplished by the rearof the speaker diaphragm, Iacting through the larger area of the mouth32.

In this respect, this invention is an improvement over couplersheretofore available. In the bass-reex couplers a similar lowerresonance occurs, but the radiation is limited by the relatively smallsize of the port.

In Figs. 5 and 6 there is shown in more detail a preferred form ofdamping ring 21 which is preferably employed in connection with thespeaker 20 and consists of a ring 40 of glass wool, mineral wool,textile bers or the like, snugly fitted within a truncated cone orcylinder 41 which serves as a cowl. The cylinder 41 is composed of anysuitable material for retaining the glass wool ring 40 in positionaround the dust cover 42 of the speaker 20.

Referring now more particularly to Fig. 7 in which frequencies areplotted as abscissas, and impedances expressed in decibles are plottedas ordinates, the curve designated at A is a typical curve for an eightinch permanent magnet speaker 20 having a free air cone resonance of 68c.p.s. The curve designated at B shows the relative electrical impedanceof the speaker 20 plotted against frequency with the cowl 41 attached,but without any padding ring 40 employed. The air mass within the cowl41 has a loading effect which, as indicated, has effected a reduction ofthe resonance frequency to 60 c.p.s.

The curve designated at C shows the impedance effect with a one inchthick ring of glass wool 40 snugly tted within the cowl 41 and providesa considerable drop Yin the impedance peak. The curve designated as Dshows the impedance with a glass wool ring 40 of ya thickness of twoinches within the cowl 41 and with a further reduction in the impedancepeak.

It will be noted that the Q of the speaker, which is the measure ofvibratory energy stored against vibratory energy expended, has beenlowered in a ratio of more than four to one, so that transient hangoverin the low frequency range is repressed.

The curve shown at E gives the corresponding electrical impedance withinthe air coupler, and it will be noted that a dipoccurs at speakerresonance caused by the quarter wave air column antiresonance. Theentire impedance variation is held to about two decibels through thetirst four octaves.

The damping ring 21 shown particularly in Figs. 5 and 6 has anadditional purpose of maintaining an imp pedance match as the radiationYresistance at. the mouth 32 falls -away with the frequency. Theinsertion Vof the damping ring 21, diagrammatfi'callyV illustratedl atRd in Fig. 8, places at the driver end of the transmission network afrequency dependent resistance similar to that imposed on thel mouth 32by limitation of. size*d ./TheV damping ring 21 has a stillfuxtherzpurpose in assisting in discriminating against mid` and: high`range frequency response from the back of the speaker 20. n

1. An acoustical device comprising. azlouds'peaker and an enclosure forsaid loudspeaker; said enclosure having a plurality of spaced side wallpanels connected along their meeting margins, an 'endf wall panelclosing said side wall panels at one endthereof; a second end `wallpanel closing part of the opposite-endet said side wall panels, a battleconnected; to and extending inwardly from said second end wallv panel;to an* end margin beyond the loudspeaker and providing on onev sidethereof with certain of said side wall panels a mouth portion and on theother side thereof with certain. of said side wall panels and -saidsecond end wall panel anvinterior throat portion, said side wall panelsat said'. oppositeY end. terminating in the same planeN as said second,end Wall panel and providing the terminus; of the mouth portion coplanarwith said second end wall panel;Y said bale having van openingtherethrough, andsaidiloudspeaker being mounted at said opening with onediaphragm surface facing into said throat portion and one diaphragmsurface facing across said mouth portion;v saidsidewall panels, bale andend wall panels providing an air column extending from the throatportion to the terminus of the mouth portion Iand folded back uponitselfat said first end wall panel, the length of said airl column -being notmore than a quarter wave at the primary resonant frequency of theloudspeaker; said side Wall panels between the end margin of said baffleand said end wall panel providing an energy sinkv of larger volume thaneither the throat volume or the mouth volume.

2. An acoustical device as dened in claim 1 in which the rear of theloudspeaker is provided with a pad of fibrous material. Y

3. An acoustical device as dened in claim 1` in which one of the sidewalls in facing relation to the opening in the bale has a hard surfacereilector plate mounted thereon.

References Cited in the tile of this patent UNITED STATES PATENTS2,217,279 Karns Oct. 8, 1940 2,440,078 Devine Apr. 20, 1948 2,622,693Leslie Dec. 23, 1952 2,765,864 Glenn Oct. 9, 1956 2,801,704 Martin Aug.6, 1957' 2,871,971 Beecroft et al. Feb; 3, 1959' FOREIGN PATENTS 747,263Great Britain Mar. 28, 1956

